Contacts to n-GaAs doped-substrates resulting in either Ohmic or rectifying behavior are technologically important for use in electronic and optoelectronic devices and circuits. Ideally, an Ohmic contact should allow the required current with a voltage drop that is substantially small compared to the drop across the active region of the device, so as not to significantly disturb device operation. During fabrication of GaAs devices, annealing temperatures routinely reach about 400.degree. C., and can be as high as 600-800.degree. C. during solar cell and self-aligning device processing. A key issue in manufacturing such devices is producing thermally stable and reliable contacts.
For manufacturing contacts to n-GaAs doped substrates, no elemental metal offers a low Schottky barrier which would avoid undesirable rectifying behavior. The most commonly used Ohmic contact to n-GaAs material are systems based on the Au-Ge eutectic. When an Au-Ge film on GaAs doped material is heated to the eutectic temperature (356.degree.), an Ohmic contact is formed. Au-Ge, however, does not easily wet GaAs. To promote wetting, small amounts of Ni (from 2-11 wt %) are added during deposition. Many other schemes of Ohmic contact formation have been conceived. However, most require metallurgical interaction with the GaAs (alloying) induced by a high temperature (often &gt;400.degree. C.) thermal anneal, a time-consuming process often resulting in undesirable contact morphology.
Other techniques, have been attempted without particular success. For example, sequential sputtering of Co and Ge followed by an anneal to produce Ge-rich Co-Ge contacts has been carried-out. However, the anneal step was shown to induce chemical interaction with the GaAs, and in most cases produces rectifying behavior for the contacts. The resulting product is entirely unsuitable as a contact in a semiconductor device.
In summary, the conventional art, in particular that using Au-Ge eutectic to make ohmic contact to GaAs, suffers from lack of temperature stability resulting in a lack of film/contact uniformity. The alloying process produces a non-uniform contact with the substrate. Further, undesirable interactions with the substrate occur, as well as other drawbacks illustrated.